Cladding-pumped fiber laser devices are important in a wide variety of optical applications. For example, in optical communications, cladding-pumped lasers are used as an input optical source for high power Erbium/Ytterbium (Er/Yb) amplifiers, remotely located Erbium (Er) amplifiers in repeaterless communications systems, as well as Raman lasers. In addition, cladding pumped fiber laser devices have promising applications as light sources for printers, in medical optics, and in material processing and manufacturing.
A typical cladding pumped fiber comprises a single-mode core and inner and outer cladding layers. The inner cladding layer surrounding the core is typically a silica cladding having a relatively large cross-sectional area (as compared to the core) and a relatively high numerical aperture. It is usually non-circular (rectangular or star-shaped) to ensure that the modes of the inner cladding may overlap with the core. The outer cladding is commonly composed of a relatively low refractive index polymer. The index of the core is greater than that of the inner cladding, which, in turn, is greater than that of the outer cladding.
A major advantage of the cladding pumped fiber is that it is able to convert light from a relatively low-brightness source into light of a relatively high brightness in a single-mode fiber. Light from low-brightness sources such as diode arrays may be coupled into the inner cladding due to its relatively large cross-sectional area and relatively high numerical aperture. In a cladding pumped fiber laser, the core is doped with a rare earth metal such as Er. The light in the inner cladding interacts with the core and is absorbed by the rare earth dopant. Therefore, if optical feedback is provided (e.g., as by employing a Bragg grating optical cavity), the cladding pumped fiber will act as a laser oscillator at the feedback wavelength.
One difficulty preventing the further exploitation of cladding pumped fiber devices is efficiently coupling a sufficient number of low-brightness source(s) into the inner cladding. A common approach is to couple the light from broad-stripe semiconductor lasers into multi-mode fibers, bundle the fibers, and then to use bulk optics to couple the light from the bundle into the cladding pumped fiber. See, for example, U.S. Pat. No. 5,268,978. The difficulty with this approach, however, is that it requires a number of fine interfaces with associated problems of matching and alignment, as well as two sets of fiber optics. An astigmatic lens is typically disposed between the semiconductor lasers and the bundling fibers and between the bundling fibers and the fiber laser. Polishing, antireflection coatings and maintenance of precise alignments are also required.
Another difficulty preventing the further exploitation of cladding pumped fiber lasers is in coupling multi-mode pump light into the inner cladding while simultaneously coupling single-mode light out of or into the single-mode core. Various solutions have been proposed to overcome this limitation. See, for example, U.S. Pat. No. 4,815,079. One known solution proposes an optical fiber laser in which radiation is pumped into a single-mode core of a laser cavity via a double clad fiber coupling arrangement. A first multi-mode layer surrounds a single-mode core, and it, in turn, is surrounded by a second outermost layer. The multi-mode layer is a core with respect to the outermost layer, which serves as a cladding. At the same time, the multi-mode layer is a cladding with respect to the single-mode core. Pump light coupled into the first multi-mode cladding, either through its end facet or its side, propagates along its length, undergoing a multiplicity of reflections at its core-cladding interface, while intersecting and being absorbed by the innermost core to cause lasing action. In one type of fiber laser, the multi-mode cladding takes the form of a rectangular slab that extends along the laser length.
Fiber lasers are usually made fairly long, on the order of thirty meters or so, with small cross-sectional geometries to make them easy to coil into compact configurations and prevent concentration quenching. However, these qualities, while advantageous for a variety of reasons, make it extremely difficult to efficiently couple high pump power into them to promote lasing. Those skilled in the art have addressed the foregoing problem with more or less success in a variety of ways.
Various solutions have been proposed to overcome the foregoing problem. See, for example, U.S. Pat. Nos. 4,818,062 and 5,127,068. In one known solution, a fiber laser is employed with a bundle of waveguides that receive light from a set of diode laser elements and couple their outputs into a solid state laser medium through a lens. While such an arrangement is adequate for solid state lasers having entrance facets with dimensions of several millimeters, it is more difficult to achieve efficient coupling of high power from the waveguide bundle into the much smaller entrance ends of long, thin, low loss, single-mode fiber lasers with multi-mode claddings. Another disadvantage of this arrangement lies in the difficulty one encounters in matching the lateral and transverse numerical apertures of the waveguides with the lateral and transverse divergences of the laser elements.
In another solution, a cylindrical microlens is employed for collimating the high numerical aperture (NA) output emissions of laser diode arrays so that such emissions may be coupled into an optical fiber. The microlens has a diameter roughly equal to the diameter of the fibers and 20% to 50% bigger than the lateral dimension of the laser diode array. An elliptical or hyperbolic cross-sectional shape may prove useful for correction of particular spherical aberrations. Such an arrangement requires a precisely small spacing from the microlens to the optical fibers, creating a crowded condition in which unintended contact and damage to the microlens or fibers may occur. Moreover, positional and alignment errors may prevent matching of the numerical apertures of the fibers with the divergences of the laser diodes, thus tending to reduce coupling efficiency.
In known laser arrangements, the pump is coupled to the laser outside of the laser cavity. Accordingly, the typical configuration of a known laser includes a pump combiner connected to a first grating, which is connected via a length of fiber to a second grating, wherein the gratings define the outer boundaries of the laser cavity. See, for example, U.S. Pat. No. 5,268,978. Additionally, the gratings that are utilized with such prior art laser configurations typically require a relatively low index coating to support both multi-mode pumping and single-mode signal light.
It is apparent that a need exists for an improved coupling system for pumping a solid state medium of a fiber laser cavity, which avoids the cost, complexity and other problems associated with the conventional approaches. For example, there is a need for a new robust and compact arrangement for coupling the output of low-brightness sources into cladding pumped fibers. In addition, there is a need for an efficient means of simultaneously coupling multi-mode pump light into the inner cladding of a cladding pumped fiber while coupling single-mode light into or out of the core.